1. Field of the Invention
The present invention relates to a method for automatically measuring a thickness of a ball bonded to a pad of a semiconductor chip in wire bonding which attains wiring between the pad of the semiconductor chip and an external electrode by utilizing image processing.
2. Description of the Related Art
Referring first to FIGS. 2A and 2B, a bonded condition between a pad of a semiconductor chip and a ball in wire bonding will be described.
In FIGS. 2A and 2B, reference numeral 1 denotes a lead frame; 2 denotes a semiconductor chip mounted on the lead frame 1; 3 denotes a pad formed on the semiconductor chip 2 (a first bonding point); 4 denotes a bonding wire such as a gold wire; and 5 denotes a ball formed at the tip of the bonding wire 4 and bonded to the pad 3.
The ball 5 formed at the tip of the bonding wire 4 has an almost spherical shape before bonding and the ball 5 is pressedly bonded onto the pad 3 by applying supersonic vibration while pressing with a capillary 6 at the time of wire bonding, resulting in the bonded ball 5 being deformed or flattened as shown in FIG. 2A.
At the center of the top surface of the ball 5 bonded to the pad 3, a cone portion 8 with a conical shape is formed by an inside chamfer portion 7 recessed into the inside of the tip of the capillary 6, and an almost flat upper surface 9 is formed around the cone portion 8 of the ball 5.
As a measuring object of the present invention, the thickness of the bonded ball is a height t from a surface of the pad 3 to the flat upper surface 9 of the bonded ball 5.
Conventionally, when the thickness t of the bonded ball 5 is measured, the thickness t of the ball 5 is measured by the use of a metallographic microscope which can measure displacement in a Z-axis direction or a vertical direction by an operator manually focusing the microscope on each of the surface of the pad 3 and the upper surface 9 of the bonded ball 5, to thereby obtain a distance between two points in focus. In the case of such a manual measuring method, it is difficult to obtain a sufficient measurement accuracy because variations are generated depending on the operator who judges whether the microscope is accurately focused on the pad 3 and the upper surface 9 of the bonded ball 5.
Although another measuring method utilizing a laser beam is known in the art, since a width of the upper surface 9 of the bonded ball 5 has an extremely small dimension of a few microns, a problem is posed in that a system irradiating the laser beam to a spot in such a microscopic location will be considerably expensive.
In order to solve the problem described above, a method is proposed to measure the thickness t of the bonded ball 5, wherein a plane image of the cone portion 8 of the ball 5 captured through a lens of a camera is compared and checked with a reference pattern image of the cone portion which is set in advance and an image taking height position at which the images are best matched is regarded as an in-focus height (Japanese Patent Application Laid-Open Publication No. 6-224267).
However, in the case of the measuring method described in Japanese Patent Application Laid-open Publication No. 6-224267, since a shape and a size of a cone portion 8 is varied by a capillary 6 used, a pattern image of the cone portion 8 must be rerecorded each time the capillary 6 is replaced, resulting in cumbersome handling of an apparatus. Moreover, since the reference pattern image of the cone portion 8 is set manually, the setting varies between individuals, and a measurement result may change depending on how the pattern image is set.